This invention relates to noise cancelling headsets (e.g., headphones, ear buds, etc.).
Noise cancellation headsets are used in, among other places, high-noise environments such as aircraft cockpits or in the vicinity of loud machines. A variety of techniques have been developed to provide noise cancellation in headsets. For example, many conventional noise cancellers use analog noise cancellation, and use either feedback or feed-forward control techniques. Feedback noise cancellation is commonly used in headsets with large acoustic cavities. Feed-forward noise cancellation is commonly used in ear buds and on-ear headsets.
Feed-forward noise cancellers cancel unwanted ambient noise signals arriving at a wearer's ear using the principle of superposition. For example, feed-forward noise cancellers generate anti-noise signals using a canceller filter that is based on a plant model (e.g., a transfer function) for the headset. Particularly, the cancellers create anti-noise signals which are equal or approximately equal in magnitude, and opposite in phase (i.e., approximately 180° out of phase), to cancel the unwanted noise signals. This is achieved using a reference microphone. The reference microphone is placed on the outside or periphery of a headset, and senses incoming unwanted noise signals. The sensed noise signals are processed and, using the plant model, the anti-noise signal is generated.
Conventionally, the plant is determined using empirical methods. In order for the analog noise canceller to provide optimal performance, the canceller filter must be finely tuned to match the dynamics of the actual headset. This is achieved, for example, by changing or updating parameters of the canceller filter while monitoring its performance. However, in order to correctly generate anti-noise signals, the noise canceller must be able to accurately identify noise signals at the wearer's ear while the headset is being worn. A loudspeaker is then used to drive both the normal audio signals and the anti-noise signals.
An example of an analog feed-forward noise canceller system is shown in FIG. 1. The system 10 includes a reference microphone 15, a speaker 20, and a feed-forward controller 25. An audio signal, x(t), is a signal from an audio device, and an acoustic signal, y(t), is a signal at the wearer's ear. The headset plant model is determined from d(t) and y(t). However, a secondary path also exists which affects noise cancellation. An example of a feed-forward system 30 which includes an error microphone 35, a secondary path model 40, an adaptation module 45, and a canceller filter 50 is illustrated in FIG. 2. When the error microphone 35 is used, the plant model refers to a transfer function between the reference microphone 15 and the error microphone 35, and the secondary path generally refers to the path between the speaker 20 and the error microphone 35. Accurate identification of the secondary path's transfer function is necessary to correctly update the canceller filter.